arXiv subjects
Publications and source records attributed to Rongwei Hu.
We report the single crystal growth of Ca(Fe1-xCox)2As2 (0 <= x <= 0.082) from Sn flux. The temperature-composition phase diagram is mapped out based on the magnetic susceptibility and electrical transport measurements. Phase diagram of Ca(Fe1-xCox)2As2 is qualitatively different from those of Sr and Ba, it could be due to both the charge doping and structural tuning effects associated with Co substitution.
The upper critical fields, Hc2 of single crystals of Sr1-xEux(Fe0.89Co0.11)2As2(x=0.203 and 0.463) were determined by radio frequency penetration depth measurements in pulsed magnetic fields. Hc2 approaches the Pauli limiting field but shows an upward curvature with an enhancement from the orbital limited field as inferred from Werthamer-Helfand-Hohenberg theory. We discuss the temperature dependence of the upper critical fields and the decreasing anisotropy using a two-band BCS model.
The lattice dynamics of FeSb2 is investigated by the first-principles DFT calculations and Raman spectroscopy. All Raman and infra-red active phonon modes are properly assigned. The calculated and measured phonon energies are in good agreement except for the B3g symmetry mode. We have observed strong mixing of the Ag symmetry modes, with the intensity exchange in the temperature range between 210 K and 260 K. The Ag modes repulsion increases by doping FeSb2 with Co. There are no signatures of the electron-phonon interaction for these modes.
We present critical fields, thermally-activated flux flow (TAFF) and critical current density of tetragonal phase beta-FeSe single crystals. The upper critical fields Hc2(T) for H||(101) and H\bot(101) are nearly isotropic and are likely governed by Pauli limiting process. The obtained large Ginzburg-Landau parameter k \sim 72.3(2) indicates that beta-FeSe is a type-II superconductor with smaller penetration depth than in Fe(Te,Se). The resistivity below Tc follows Arrhenius TAFF behavior. For both field directions below 30 kOe single vortex pinning is dominant whereas collective creep becomes important above 30 kOe. The critical current density Jc from M-H loops for H||(101) is about five times larger than for H\bot(101), yet much smaller than in other iron-based superconductors.
Understanding iron based superconductors requires high quality impurity free single crystals. So far they have been elusive for beta-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic impurity phases. Herein we report synchrotron - clean beta-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect induced weak ferromagnetism that coexists with superconductivity below Tc. In contrast to Fe1+yTe - based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).
We report inelastic neutron scattering measurements aimed at investigating the origin of the temperature-induced paramagnetism in the narrow-gap semiconductor FeSb2. We find that inelastic response for energies up to 60 meV and at temperatures 4.2 K, 300 K and 550 K is essentially consistent with the scattering by lattice phonon excitations. We observe no evidence for a well-defined magnetic peak corresponding to the excitation from the non-magnetic S = 0 singlet ground state to a state of magnetic multiplet in the localized spin picture. Our data establish the quantitative limit of S_{eff}^2 < 0.25 on the fluctuating local spin. However, a broad magnetic scattering continuum in the 15 meV to 35 meV energy range is not ruled out by our data. Our findings make description in terms of the localized Fe spins unlikely and suggest that paramagnetic susceptibility of itinerant electrons is at the origin of the temperature-induced magnetism in FeSb2.
Kondo insulator like material FeSb2 was found to exhibit colossal Seebeck coefficient. It would have had huge potential in thermoelectric applications in cryogenic temperature range if it had not been for the large thermal conductivity. Here we studied the influence of Te doping at Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. Surprisingly, only 5% Te doping suppresses thermal conductivity by two orders of magnitude, which may be attributed to the substitution disorder. Te doping also results in transition from an semiconductor to a metal. Consequently thermoelectric figure of merit (ZT ? 0:05) in Fe(Sb0:9Te0:1)2 at ? 100K was enhanced by about one order of magnitude when compared to ZT < 0:005 in undoped FeSb2.
We have measured polarized Raman scattering spectra of the Fe$_{1-x}$Co$_{x}$Sb$_{2}$ and Fe$_{1-x}$Cr$_{x}$Sb$_{2}$ (0$\leq x\leq $0.5) single crystals in the temperature range between 15 K and 300 K. The highest energy $B_{1g}$ symmetry mode shows significant line asymmetry due to phonon mode coupling width electronic background. The coupling constant achieves the highest value at about 40 K and after that it remains temperature independent. Origin of additional mode broadening is pure anharmonic. Below 40 K the coupling is drastically reduced, in agreement with transport properties measurements. Alloying of FeSb$_2$ with Co and Cr produces the B$_{1g}$ mode narrowing, i.e. weakening of the electron-phonon interaction. In the case of A$_{g}$ symmetry modes we have found a significant mode mixing.
The vibrational properties of ErTe$_3$ were investigated using Raman spectroscopy and analyzed on the basis of peculiarities of the RTe$_3$ crystal structure. Four Raman active modes for the undistorted structure, predicted by factor-group analysis, are experimentally observed and assigned according to diperiodic symmetry of the ErTe$_3$ layer. By analyzing temperature dependence of the Raman mode energy and intensity we have provided the clear evidence that all Raman modes, active in the normal phase, are coupled to the charge density waves. In addition, new modes have been observed in the distorted state.
Intrinsic, in-plane anisotropy of electrical resistivity was studied on mechanically detwinned single crystals of SrFe$_2$As$_2$ above and below the temperature of the coupled structural/magnetic transition, $T_{\textrm{TO}}$. Resistivity is smaller for electrical current flow along the orthorhombic $a_o$ direction (direction of antiferromagnetically alternating magnetic moments) and is larger for transport along the $b_o$ direction (direction of ferromagnetic chains), which is similar to CaFe$_2$As$_2$ and BaFe$_2$As$_2$ compounds. A strongly first order structural transition in SrFe$_2$As$_2$ was confirmed by high-energy x-ray measurements, with the transition temperature, and character unaffected by moderate strain. For small strain levels, which are just sufficient to detwin the sample, we find a negligible effect on the resistivity above $T_{\textrm{TO}}$. With the increase of strain, the resistivity anisotropy starts to develop above $T_{\textrm{TO}}$, clearly showing the relation of anisotropy to an anomalously strong response to strain. Our study suggests that electronic nematicity cannot be observed in the FeAs based compounds in which the structural transition is strongly first order.
Thermally activated flux flow (TAFF) and flux flow Hall effect (FFHE) of Fe(Te,S) single crystal in the mixed state are studied in magnetic fields up to 35 T. Thermally activated energy (TAE) is analyzed using conventional Arrhenius relation and modified TAFF theory which is closer to experimental results. The results indicate that there is a crossover from single-vortex pinning region to collective creep pinning region with increasing magnetic field. The temperature dependence of TAE is different for H//ab and H//c. On the other hand, the analysis of FFHE in the mixed state indicates that there is no Hall sign reversal. We also observe scaling behavior |Rhoxy(H)| = A*Rhoxx(H)^beta.
In this work we investigated the temperature dependence of the upper critical field u0Hc2(T) of Fe1.02(3)Te0.61(4)Se0.39(4) and Fe1.05(3)Te0.89(2)Se0.11(2) single crystals by measuring the magnetotransport properties in stable dc magnetic fields up to 35 T. Both crystals show that u0Hc2(T) in the ab-plane and along the c-axis exhibit saturation at low temperatures. The anisotropy of u0Hc2(T) decreases with decreasing temperature, becoming nearly isotropic when the temperature T->0. Furthermore, u0Hc2(0) deviates from the conventional Werthamer-Helfand-Hohenberg (WHH) theoretical prediction values for both field directions. Our analysis indicates that the spin-paramagnetic pair-breaking effect is responsible for the temperature-dependent behavior of u0Hc2(T) in both field directions.
We have investigated the upper critical field anisotropy and magnetotransport properties of Fe1.14(1)Te0.91(2)S0.09(2) single crystals in stable magnetic fields up to 35 T. The results show that u0Hc2(T) along the c axis and in the ab-plane exhibit saturation at low temperatures. The anisotropy of u0Hc2(T) decreases with decreasing temperature, becoming nearly isotropic for T->0. Our analysis indicates that the spin-paramagnetic pair-breaking with spin-orbital scattering is responsible for the behavior of u0Hc2(T). Furthermore, from analysis of the normal state properties, we show evidence that the excess Fe acting as Kondo-type impurities is a key factor determining the normal and superconducting state physical properties.
We have used optical spectroscopy to probe the normal state electrodynamic response of Fe$_{1.06}$Te$_{0.88}$S$_{0.14}$, a member of the 11 family of iron-based superconductors with T$_c$= 8 K. Measurements have been conducted over a wide frequency range (50 - 50000 cm$^{-1}$) at selected temperatures between 10 and 300 K. At low temperatures the material behaves as an "incoherent metal": a Drude-like peak is absent from the optical conductivity, and all optical functions reveal that quasiparticles are not well defined down to the lowest measured temperature. We introduce "generalized spectral weight" analysis and use it to track temperature induced redistribution of spectral weight. Our results, combined with previous reports, indicate that the 11 family of iron-based superconductors might be different from other families.
The in-plane London penetration depth, $λ(T)$, was measured in single crystals of the iron-chalcogenide superconductors Fe$_{1.03}$(Te$_{0.63}$Se$_{0.37}$) and Fe$_{1.06}$(Te$_{0.88}$S$_{0.14}$) by using a radio-frequency tunnel diode resonator. As is also the case for the iron-pnictides, these iron-chalcogenides exhibit a nearly quadratic temperature variation of $λ(T)$ at low temperatures. The absolute value of the penetration depth in the $T \to 0$ limit was determined for Fe$_{1.03}$(Te$_{0.63}$Se$_{0.37})$ by using an Al coating technique, giving $λ(0)\approx560 \pm 20$ nm. The superfluid density $ρ_s(T)=λ^2(0)/λ^2(T)$ was fitted with a self-consistent two-gap $γ-$model. While two different gaps are needed to describe the full-range temperature variation of $ρ_s(T)$, a non-exponential behavior at low temperatures requires additional factors, such as scattering and/or significant gap anisotropy.
We report synthesis of high quality Fe1+y(Te1-xSx)z single crystals and a comprehensive study of structural, magnetic and transport properties. There is high sensitivity to material stoichiometry which includes vacancies on the Te(S) site. Our results reveal competition and coexistence of magnetic order and percolative superconductivity for x >= 0.03, while zero resistivity is acheived for x >= 0.1.
We report single crystal growth and a study of superconducting properties in Fe_{1+y}Te_{1-x}S_{x}. We demonstrate the smallest upper critical field anisotropy, gamma=H_{c2}^{//c}/H_{c2}^{//a}, among all iron based superconductors, the value of gamma reaches 1.05 at T=0.65T_{C} for Fe_{1.09}Te_{0.89}S_{0.11}, while still maintaining large values of upper critical field.
We report results of our infrared and optical spectroscopy study of a half-metallic ferromagnet Mn$_5$Ge$_3$. This compound is currently being investigated as a potential injector of spin polarized currents into germanium. Infrared measurements have been performed over a broad frequency (50 - 50000 cm$^{-1}$) and temperature (10 - 300 K) range. From the complex optical conductivity $σ(ω)$ we extract the electron self-energy $Σ(ω)$. The calculation of $Σ(ω)$ is based on novel numerical algorithms for solution of systems of non-linear equations. The obtained self-energy provides a new insight into electron correlations in Mn$_5$Ge$_3$. In particular, it reveals that charge carriers may be coupled to bosonic modes, possibly of magnetic origin.